1. Field of the Invention
This invention relates to a method of preventing the deformation of a refrigerated gas pipeline which traverses frost-susceptible soil. In particular, the invention pertains to a method utilizing refrigerating-type heat pipes installed below the pipeline to remove heat from the soil without regard to seasonal temperature variations, thereby preventing excessive frost heaving of the pipeline.
2. Description of the Prior Art
Permafrost often has a high water content so that if it becomes thawed to any significant extent, it is unable to adequately support structures on or in it. Heat pipes have been used previously in connection with support piles for pipelines and other structures in order to stabilize the soil in arctic regions where permafrost is prevalent. For example, U.S. Pat. No. 3,859,800 (issued to L. E. Wuelpern on Jan. 14, 1975) teaches the use of piles passively refrigerated by heat pipes for permafrost stabilization of elevated pipelines, and U.S. Pat. No. 3,788,389 (issued to E. D. Waters on Jan. 29, 1974) shows the use of heat pipes for stabilizing soil surrounding structural supports (such as telephone poles).
A different problem exists with refrigerated gas pipelines used to transport arctic gas. These pipelines are refrigerated where they pass through permafrost in order to prevent thaw-settlement of the pipeline and to prevent soil erosion, icings, slope instability and other problems related to thawing permafrost. Also, there are economic incentives for chilling gases in the currently proposed large diameter, high pressure pipelines due to higher gas density, lower flowing pressure losses and lower compression costs at lower temperature. A discussion of refrigerated gas pipelines is given by G. King, "The How and Why of Cooling Arctic Gas Pipelines", Parts I and II, Pipeline and Gas Journal (September and October, 1977).
When these refrigerated pipelines traverse unfrozen ground or shallow permafrost where the soil is frost-susceptible, damage due to frost heaving is possible. Frost heaving can occur when water migrates toward the cold pipeline, collecting in layers of almost pure ice (ice lenses) beneath the pipeline. The resulting extra volume of ice causes soil deformation, usually in the form of heaving of the soil and pipeline above the lenses. The pipe may be heaved out of the ground in some cases. Moreover, the possibility of differential heave magnifies the threat to pipeline integrity. Differential heave occurs where the pipeline passes through adjacent soil zones that heave at different rates. For example, the pipeline may encounter a region of unfrozen frost-susceptible ground surrounded by permafrost. When this unfrozen ground freezes due to cooling by the pipeline, it will heave much more rapidly than the surrounding permafrost. The resulting differential heave can cause wrinkling and, ultimately, rupture of the pipeline.
Several methods have been proposed for dealing with the problem of frost heave of refrigerated gas pipelines, including replacing the frost-susceptible soil surrounding the pipeline with non-frost-susceptible soil and physically restraining the pipeline to prevent heave. Another solution proposed in the prior art has been to heavily insulate the pipeline and/or heat the soil beneath the pipeline in order to prevent formation of the ice lenses. A discussion of the problems and current approaches for operating refrigerated gas pipelines in permafrost and unfrozen soil may be found in A. C. Matthews, "Natural Gas Pipeline Design and Construction in Permafrost and Discontinuous Permafrost", SPE 6873 (1977).
While these methods provide some measure of relief from the problems of frost heaving, there are serious difficulties associated with each. These methods generally will involve specialized construction techniques, as where the pipelines are coated with insulation material and where individual electric heaters are installed. Careful surveillance and frequent adjustments of heating rates are also required. Further, adequate methods for monitoring pipeline heave are not yet available. Finally, these specialized techniques and devices inherently involve very high, possibly prohibitive costs due to the great length of a pipeline system requiring frost heave protection. Hundreds of transitions from frozen to thawed ground may be encountered with any major arctic gas pipeline. For example, precautions will be taken to protect the Alaska Highway Gas Pipeline from frost heave over at least an 80 mile length using some of the proposed techniques outlined above; for details, see Oilweek, page 20 (Apr. 17, 1978).
Recently, an alternative method for reducing frost heave has been suggested in U.S. patent application Ser. No. 938,662 filed on Aug. 31, 1978, now Pat. No. 4,194,856. To avoid many of the problems mentioned above, heat pipes are installed in diametrically opposed pairs, one on either side of the pipeline, spaced along the length of pipeline traversing soil subject to frost-heave. Long vertical frost bulbs are formed due to the operation of the heat pipes. These frost bulbs create confining forces on the pipeline tending to reduce frost heave. However, while this method presents advantages over methods previously proposed, a disadvantage exists in that the operation of the heat pipes is seasonal in nature, heat transfer taking place only during the winter months when the ambient temperature is colder than the soil temperature. Moreover, this places limits on constructing and operating the refrigerated pipeline because there can be a time lag before the heat pipes can operate.